The present invention relates to a method and a device for automatically estimating in real time a degradation in performance of an aircraft that can lead to negative effects on the fuel consumption and the drag of the aircraft.
More precisely, the object of the present invention is to provide a pilot of the aircraft with a numerical value of the effect of the degradation of said aircraft on the fuel consumption and/or the drag, combined with a warning when the degradation in fuel consumption and/or in drag is judged to be too high with respect to the type of mission to be accomplished.
It is known that many functions onboard an aircraft, in particular a cargo plane, use performance data. These functions notably relate to the guidance of the aircraft over a vertical flight path, or else the calculation of the predictions on consumption of fuel at the destination.
The performance calculations carried out by these functions use reference performance models. These models are formed from databases and from simplified equations of the mechanics of the flight. The databases relate to a standard aircraft, being representative of an average level of performance. The nominal fuel consumption and drag of an aircraft may be deduced from these models.
Thus, when the aircraft exhibits a degraded aerodynamic behavior or engine efficiency, the calculation of the flight path or of the fuel consumption may be optimistic and may no longer correspond to that of the real aircraft.
Accordingly, with the aim of re-adjusting the model used, correction factors from the performance of each aircraft type may be applied during the flight, such as for example penalties on the fuel consumption or on the drag. The operational procedures provided advises pilots against modifying these correction factors during the flight, except in cases for which the cause of a degradation is well identified. Moreover, the correction factors provided to the pilots are, in general, over-estimated in order to take into account the worst case scenario. Two types of possible corrections exist:                corrections which allow the aging of the aircraft to be taken into account. These factors are determined by the airline thanks to a monitoring of the fleet carried out by means of tools that the aircraft manufacturer makes available. These factors can never be modified by the pilot in flight. They are automatically loaded onto the aircraft by the airline or by an intervention of the maintenance crews; and        corrections associated with a given flight. These corrections are inserted by the pilots and are those which are considered by the present device.        
Aside from a manual monitoring of the fuel consumption, the pilots do not possess any automatic means that can assist them in estimating the impact on the performance in real time of a degradation that occurred during the flight. This manual monitoring allows the identification of the importance of the degradation after a time depending on the flightplan, but which remains in any case of the order of several tens of minutes, generally around thirty minutes.
Thus, as regards the fuel consumption:                the fuel predictions along the flightplan consider the aircraft to be in a state comparable to that which it was in at the start of the flight, in other words virtually nominal; and        the means for monitoring the correct level of fuel consumption during the flight are essentially manual.        
It is known that a significant degradation in the current state of the aircraft can have very significant impacts on its flight. Several cases are possible, and notably:                A/ during a civilian mission, the pilot becomes aware of a degradation of the current state of the aircraft. The real degradation from the fault is, in general, over-estimated by the means available to the pilot, which may lead to a diversion. This situation is of no consequence for safety, but involves high costs for the airline;        B/ during a civilian mission, the pilot becomes aware very late of a degradation of the current state of the aircraft;        C/ during a mission following a pre-calculated flight path, a degradation occurs. The pilot becomes aware of a degradation when the latter generates an effect to which it is sensitive, such as for example a decrease in the margin with respect to the stall or a reduction in its climbing capacities. Certain types of degradation take effect in a gradual manner, when the aircraft is still far from its limits. If the automatic pilot system is active, the pilot does not necessarily notice a change in the behavior of the aircraft. He will have to wait until the maximum performance is requested in order to become aware that the aircraft is degraded; and        D/ during a civilian mission, the cruising phase may be reduced to a minimum. Over the majority of the flight, the aircraft will be climbing or descending, will undergo modifications in true airspeed, will engage turns in order to follow its route and will be subjected to the atmospheric conditions. There is a risk of never (or very rarely) encountering stable conditions, which will accordingly delay the information on the situation to the pilot.        